The core of a modern axial flow gas turbine engine is typically enclosed by a casing that comprises a number of coaxial annular cross-section portions that are connected in series. Conventionally, each of the casing portions is provided with a radially outwardly extending annular flange at each of its axial extents. The flanges of adjacent casing portions are maintained in engagement with each other by bolts that extend through suitably located and aligned holes is in the flanges.
In the compressor and turbine sections of such an engine, annular arrays of rotor blades are contained within the casing. During engine operation, these rotor blade arrays rotate at very high speeds. If any of the rotor blades should suffer a structural failure, they travel at high speed in a generally radially outward direction to impact the casing section adjacent thereto. In the interests of safety, the casing must be sufficiently strong to contain such failed blades. However, this is a more difficult task if the failed blades are located adjacent the previously mentioned flanges joining adjacent two casing portions. If one of the casing portions is less rigid than the other, there can be a tendency for that casing portion to be radially deflected to a greater extent than the other. This brings about the imposition of shear loads upon some of the bolts joining the casing portions and their eventual failure. High pressure gases normally present within the casing then tend to urge the flanges apart, so in turn exerting tensile loads upon the remaining bolts and in some cases, causing their failure also.
If adjacent casing section flanges are urged apart in this manner, there is, of course, an increased likelihood of the non-containment by the casing of failed rotor blades and associated debris.
One way in which this problem could be tackled is by the strengthening of one of the casing portions so that its rigidity is increased to a level similar to that of the casing portion adjacent thereto. However, this results in a significant increase in the weight of the casing portion which is a undesirable feature in aircraft-mounted gas turbine engines.